The work proposed here focuses on two chaperone-like proteins that play critical roles in the biogenesis of small RNAs. The 60kDa Ro protein is part of a quality control system for small RNAs;it recognizes misfolded variant pre-5S rRNAs and U2 RNAs and targets them for degradation. Ro protects the cell against UV damage, and its proper function is necessary to protect against autoimmune disorders. The approximately 50kDa La is the first protein to associate with many RNA polymerase III transcripts immediately after their synthesis and facilitates their correct maturation and targeting. Additionally, it has been shown to assist in the correct folding of certain pre-tRNAs in vivo. Our first aim is to determine the crystal structure of the Ro protein and to investigate by mutagenesis how it interacts with RNA. Diffraction quality crystals have been obtained in preliminary studies. The second aim is to determine crystal structures of Ro and RNA in order to understand how Ro recognizes misfolded small RNAs and how Ro modulates its affinity for different RNAs. Mutagenesis studies will be used to investigate which protein-RNA interactions are most important in determining the affinity and specificity of Ro for RNAs. The third aim is to determine a crystal structure for the La protein, preferably bound to RNA, as the first step in investigating how La interacts with RNA. All three aims are directed at understanding how proteins interact with RNAs at the atomic level. Additionally, since both Ro and La are major autoantigens for the autoimmune diseases Sjogren's syndrome and systemic lupus erythematosus, their three-dimensional structures may be useful in determining how they trigger an autoimmune response.